This invention relates to a process for preparing hydroxy-functionalized polyesters.
When manufacturing polymers, it is desirable to be able to control the molecular weight. Unfortunately, polymers produced from the reaction of diepoxides and diacids, such as adipic acid, exhibit the tendency to increase in molecular weight over time, when held at polymerization temperatures. This can be a problem because it may take several hours to empty a reactor contents during which time the molecular weight of the polymer increases. This molecular weight increase occurs even when standard stoichiometric calculations suggest that no molecular weight build should be taking place.
It is known to control the molecular weight by adding a large excess of a mono-functional acid to the reactor contents when the desired molecular weight has been achieved. The disadvantage here is that the excess acid may need to be removed or the material of construction for the equipment associated with production may not be compatible with the acids used.
U.S. Pat. No. 5,171,820 describes a process for preparing hydroxy-functionalized polyesters by allowing a dicarboxylic acid and a diglycidyl ether to react in a solvent containing a quaternary ammonium halide as an initiator. This patent teaches that a monofunctional carboxylic acid can be added at the end of the polymerization reaction. This end-capping step does prevent the molecular weight build that might otherwise occur during an extended heating phase of commercial production, but it requires at least 10 weight percent of the monofunctional carboxylic acid to control the molecular weight.
It would be desirable to provide a process for preparing hydroxy-functionalized polyesters in a given solvent whereby the molecular weight of the polymer can be controlled by using a lower level of a capping agent than those used in the prior art.